Pots on growbags

its not just morrisons who sell them  - your local florist will have some too!

That sounds good JayG, but even the slow release will still mean the compost directly underneath will be wet and the compost on the outside of the bags will be damp at best. After thinking today though i will be using 2L coke bottles with the lid on, upturned in the middle of the bags with the leaky hose running the length of the bag and being fed from the bottle.

I'm confused now
I thought the method for ring culture in growbags was that you cut the bottom off some florists bins rammed them into the growbag twisting as you go and cut out the depression. The bins are then pushed into the compost, a smaller bottomless bin or pot is placed inside the ring the plants are then planted in the smaller pot. Water is fed into the ring between the two pots, liquid feed is fed into the smaller pot. The idea is to keep the growbag compost wet allowing the tap roots to go where they will. The feeding roots near the top are then are then able to benefit from the liquid feed.   

the original ring culture system was a single bottomless 'ring culture' pot pushed into a deep bed of gravel or earth, which was kept wet.. You planted into compost in the pot and added feed there as well - the roots could grow into the gravel/earth for water.

Then growbags were invented and used for the bottom bed, but it was difficult to keep those evenly wet, so people started adding a small pot for watering into. Feeding was still into the top pot.

Then the double growpots were invented, plant and feed in the inside one water in the outside one.

Any set up that provides enough compost/feed and water will grow lovely tomatoes. it's up to you how you actually do it